Reduced temperature sensitivity launch motor

ABSTRACT

A rocket motor that has a main exhaust nozzle area and a secondary exhaustozzle area with the secondary exhaust nozzle area being closed by burst discs that rupture and open secondary nozzle areas when predetermined temperatures and pressures are reached when propellant is burned inside the rocket motor.

DEDICATORY CLAUSE

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto us of any royalties thereon.

BACKGROUND OF THE INVENTION

A basic problem of in-tube burning rockets is the temperaturesensitivity of the launch motor. The propellant burning rate is afunction of its ambient temperature and increases or decreases withincreases or decreases in the firing temperature. This behavior resultsin two undesirable characteristics in the launch motor design. First,the launch motor must completely burn out in the tube at the lowestfiring temperature in which the weapon will be used. This results in alonger launch tube length than that needed for the nominal temperatureoperation. Since this burning time can be at a value twice that obtainedat high temperature, the nominal burning time of the motor must becorrespondingly shorter than would be the case without a temperaturedependency. A second undesireable characteristic results from the factthat the increased burning rate at high temperatures causes higherchamber pressures which must be accommodated by added weight in themotor case and thus is responsible for further increases to total launchweight. The approach that has been taken in the past to ameliorate thisproblem is to improve on (lessen) the temperature sensitivity of thepropellant thru the use of ballistic additives. While these improvementshave resulted in temperature sensitivities that are considerably lowerthan the early propellants, propellant compromises have been made andsome temperature sensitivity still remains.

Therefore, it is on object of this invention to provide a reduction inlaunch motor temperature sensitivity thru hardware modification ratherthan propellant formulation by the incorporation of a self-regulatingcombination of nozzle throats.

Other objects and advantages of this invention will be obvious to thoseskilled in this art.

SUMMARY OF THE INVENTION

A rocket motor that has a variable area exhaust nozzle area that isprovided by having symmetrically arranged open nozzles and symmetricallyarranged burst discs that close other nozzles that are opened only whenpredetermined temperatures and pressures are reached by burningpropellant of the rocket motor to rupture the burst discs and open theother nozzles.

DESCRIPTION OF THE DRAWINGS

FIG. 1, is a perspective view of a rocket motor with main nozzles andburst disc controlled nozzles, and

FIG. 2, is a graph showing the operating characteristic of a rocketmotor with conventional nozzle structure compared to a rocket motor withnozzle structure in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, the main feature of this invention is a rocketmotor 8 with a multiple set of nozzles, some of which are closed bypressure activated burst discs 12. In the illustration of FIG. 1, thereare seven equally sized main nozzles 10 and six smaller burst disccontrolled nozzles 12 arranged in a symmetric pattern. In a lowtemperature firing, the motor chamber pressure is insufficient torupture burst discs 12 and only main nozzles 10 operate. At a highertemperature, the chamber pressure reaches a value sufficient to rupturethe burst diaphragms on three of the six burst disc controlled nozzles12. The resulting increase in throat area reduces the chamber pressureto values well below those that would occur without these additionalnozzles in operation. Further increases in the firing temperature resultin chamber pressures that become sufficiently high to rupture the burstdiscs 12 of the remaining three burst disc controlled nozzles resultingonce again in a decrease in chamber pressure. The number of main nozzlesis immaterial since it is the total area of the nozzles that control theresulting chamber pressure. The ratio of the area of main nozzles 10 tothat of the control nozzles 12 is determined by the temperaturesensitivity of the selected propellant. The number of burst disccontrolled nozzles 12 is a function of the temperature sensitivitydesired of the launch motor. The lower the temperature sensitivitydesired, the greater the number of burst discs 12 controlled nozzle setsthat must be used. In the illustration provided in FIG. 1, two sets ofthree burst disc controlled nozzles 12 are used, one set designed toopen at an intermediate pressure, and one set designed to open at aslightly higher pressure. The symmetric arrangement is choosen to avoidthe occurrence of thrust misalignment that would otherwise beencountered with a nonsymmetric nozzle flow pattern.

The operating chamber pressure of the improved structual arrangementjust discussed is shown as a function of the ambient firing temperaturein FIG. 2. The solid line represents the operating chamber pressurecurve for a conventional motor not having burst disc controlled nozzles,while the dashed line represents the operating characteristics of theimproved structure incorporating the burst disc controlled nozzles 12.The conventional structure has a fixed set of nozzles having a totalthroat area equal to the throat area of the improved structure with halfof the burst disc controlled nozzles open. By comparison, the improvedstructure operates at a higher chamber pressure than the conventionalstructure at low temperature. For this illustration, the chamberpressure of the improved structure reaches a value sufficient to rupturethe burst discs at 0° F. and the chamber pressure drops to that of theconventional structure, since the total throat area of each arrangementis equal. The operating characteristics of each arrangement remainsidentical until a temperature of 100° F. is reached, where the chamberpressure reaches a value sufficient to rupture the remaining set ofburst discs 12 and then the chamber pressure once again is reduced. Theimprovement in operating characteristic is evident in the narrowing ofthe chamber pressure range that occurs over the firing temperaturerange. In this example, a reduction in chamber pressure variation ofover 50% is achieved.

We claim:
 1. A rocket motor having a main area nozzle means forexhausting gases from the rocket motor and a secondary nozzle area meansthat is closed by burst disc means that rupture upon predeterminedtemperature and pressure being reached inside said rocket motor toinitially reduce the temperature and pressure of burning propellantwithin said rocket motor.
 2. A rocket motor as set forth in claim 1,wherein said main area nozzle means is symmetrically arranged relativeto said rocket motor and said secondary nozzle area means that is closedby said burst disc means is also symmetrically arranged relative to saidrocket motor.
 3. A rocket motor as set forth in claim 2, wherein saidburst disc means is plural in number and said burst disc means isdesigned to rupture at different temperatures and pressures to provideat least two steps of rupturing of said burst disc means to reduce thetemperature and pressure of the rocket motor twice.